BRIDGES AND
TUNNELS OF
ALLEGHENY COUNTY,
PENNSYLVANIA
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HAER
Smithfield Street Bridge, Pittsburgh, PA
01 Cover Page
02 Foreword
03 Ferries
05 Monongahela
Bridge and
Fire
12 Masonry
17 Removal
of Old and
Erection of
New Bridge
18 Flooring
19 Ornamental
Towers and
Painting
22 Alterations
23 Footnotes
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Smithfield Street Bridge, Pittsburgh, PA Historic American Engineering Record PA-2 page 11
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Smithield Street Bridge (1881) Description As in the case of Roebling, Lindenthal's is the best account of the construction of the Smithfield Street Bridge. (31) In 1881 "the writer was invited for consultation and to suggest changes in the plans, which should provide for a widening of the bridge by adding another roadway or track, should this ever become necessary in the future. After having submitting such plans, they were accepted and the writer was engaged to carry them out. This plan proposed to utilize the foundations and piers which had been commenced. They were to be built upon, without any offsets, to a width on top of 56 feet. "As the width of the superstructure may ultimately reach 64 feet, or eight feet wider than the piers, it became necessary to let the sidewalks project over the masonry. The present width is 48 feet on the channel spans; the room for widening the bridge was left on the up-stream side. For the channel spans Pauli trusses (32) were proposed, 25 feet 8 inches apart, centre to centre, and centre line of the new floor (of 48 feet width) was shifted down-stream 8 feet 2 inches from the centre line of the old bridge. The sidewalk on the up-stream side was proposed to be detachable, so that the floor may be widened and the sidewalks again connected to it. "For the approaches to the channel spans, plate girder deck spans, on lighter masonry piers, were proposed. This arrangement allows increasing the width of the bridge by simply adding more plate girders to each span on the piers which are long enough for that purpose. Being a deck bridge, it afforded an unobscured entrance view to the channel spans, the trusses of which were to rest on ornamental towers, giving to the superstructure an architectural appearance of strength and stability. "The shifting of the centre line of new floor 8 feet 2 inches down-stream from the centre line of the old bridge allowed for erection of the new superstructure without stopping travel on the old bridge, in a manner described more in detail below. "The Pauli truss type commended itself for the channel spans in this instance, for several reasons: "1. The pleasing appearance (for a city bridge) in comparison with the ordinary parallel chord truss. "2. The fact that the trusses could be made high in the middle (without detriment to their stability in case of high winds), thereby reducing the chord strains and chord sections. In connection with the light and slender web-members, it permitted the economy in the trusses of over 9 percent, as compared with parallel chord trusses (with inclined end posts) of same height (50 feet). The deflection and vibration of high trusses is small and their rigidity is great. "3. The bottom chord or cable is exposed to the sun's ray as much as any other truss member; therefore unequal temperature effects in the trusses are avoided. The covered floor construction is independent of the trusses as to temperature effects. "4. The floor had to be cambered 18 inches in each 360 foot span to agree with the general grade of the new bridge. A straight bottom chord with a rise of 18 inches in 360 feet was undesirable. "At first it was proposed to build the new structure 15 feet higher at highest point than the old bridge. But the river men, in the interests of navigation demanded the structure be at least 20 feet higher, or 57 above low water mark, to which the Bridge Company objected, on the ground that the additional 5 feet would injure travel over the bridge much more, by reason of a steep grade at the Pittsburgh end, than it would benefit navigation. "There is no statute prescribing the height of bridges over the Monongahela River. The case was taken to a court of equity, and argued there by lawyers pro and contra, resulting in a preliminary injunction against the Bridge Company building the bridge lower than 20 feet. To continue the litigation would have required much time. After a suspension of work at the bridge for 10 months the Bridge Company decided to accede to the demands of the river men. "The following is a description of the material and methods used in the construction of the bridge:
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Introduction Last modified on
30-Sep-99
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